In this article, we present an optimization study of the switchable solvent system DBU/CO2 for cellulose solubilization and derivatization via online Fourier transform infrared spectroscopy (FT-IR). By varying temperature, CO2 pressure, and solubilization time, we succeeded in achieving cellulose solubilization within 10–15 min at 30 °C. Compared to traditionally used ionic liquids, the system presented here is cheaper, is easier to recycle, and enables a very fast cellulose solubilization under mild conditions. The efficiency of our optimized mild conditions were further confirmed by X-ray diffraction (XRD) experiments showing the typical transformation from cellulose I to II upon regeneration. In addition, we prove the existence of the in situ formed carbonate anions by trapping them with benzyl bromide or methyl iodide as electrophiles, leading to the successful synthesis of cellulose benzyl carbonate and cellulose methyl carbonate, respectively, under utilization of CO2 as a renewable building block for cellulose derivatization. The synthesized cellulose carbonates were characterized by FT-IR, 1H NMR, and 13C NMR spectroscopy. A degree of substitution (DS) value of 1.06 was achieved for the cellulose benzyl carbonate as determined by 31P. This study thus provides deep insight into the possibilities of the studied switchable solvent system for cellulose solubilization and offers unprecedented possibilities for novel derivatization protocols of cellulose.< Leer menos